Summary of Non-Accrual Loans by Loan Class (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	$ 46,761	$ 53,399
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	9,462	12,195
Agriculture production financing and other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	86
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	3,786	4,814
Commercial and farm land
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	19,303	22,612
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	11,856	11,476
Home Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	2,268	1,997
Finance Lease
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans		301
Other Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans		$ 4